In essentially every process in which a mixture is prepared for a particular purpose, the constituents of that mixture usually need to be present in particular, proportionated amounts in order for the mixture to be effective for its intended use. In the aforementioned related patent applications, the underlying objective is to reduce the amount of organic solvent present in a coating formulation by the use of supercritical fluid, particularly, supercritical carbon dioxide. Understandably, with this objective in mind, it is generally desirable to utilize as much supercritical fluid as possible while still retaining the ability to effectively spray the liquid mixture of coating formulation and supercritical fluid and also obtain a desirable coating on the substrate. Accordingly, here too, it is particularly preferred that there be prescribed, proportionated amounts of supercritical fluid and of coating formulation present in the liquid coating mixture to be sprayed.
Generally, the preferred upper limit of supercritical fluid addition is that which is capable of being miscible with the coating formulation. This practical upper limit is generally recognizable when the admixture containing coating formulation and supercritical fluid breaks down from one phase into two fluid phases.
To better understand this phenomenon, reference is made to the phase diagram in FIG. 1 wherein the supercritical fluid is supercritical carbon dioxide fluid. In FIG. 1, the vertices of the triangular diagram represent the pure components of an admixed coating formulation which for the purpose of this discussion contains no water. Vertex A is a solvent, vertex B is carbon dioxide and vertex C represents a polymeric material. The curved line BFC represents the phase boundary between one phase and two phases. The point D represents a possible composition of a coating formulation in which supercritical carbon dioxide has not been added. The point E represents a possible composition of an admixed coating formulation after admixture with supercritical carbon dioxide.
Thus, after atomization, a majority of the carbon dioxide vaporizes, leaving substantially the composition of the original coating formulation. Upon contacting the substrate, the remaining liquid mixture of the polymer and solvent(s) component(s) will flow, i.e., coalesce, to produce a uniform, smooth film on the substrate. The film forming pathway is illustrated in FIG. 1 by the line segments EE'D (atomization and decompression) and DC (coalescence and film formation).
However, the amount of supercritical fluid, such as supercritical carbon dioxide, that can be mixed with a coating formulation is generally a function of the miscibility of the supercritical fluid with the coating formulation as can best be visualized by referring to FIG. 1.
As can be seen from the phase diagram, particularly as shown by arrow 100, as more and more supercritical carbon dioxide is added to the coating formulation, the composition of the admixed liquid coating mixture approaches the two-phase boundary represented by line BFC. If enough supercritical carbon dioxide is added, the two-phase region is reached and the composition correspondingly breaks down into two fluid phases. Sometimes, it may be desirable to admix an amount of supercritical fluid (in this case, supercritical carbon dioxide) which is even beyond the two phase boundary. Generally, however, it is not preferable to go much beyond this two phase boundary for optimum spraying performance and/or coating formation.
In addition to avoiding the two-phase state of the supercritical fluid and the coating formulation, proper proportionation is also desirable to provide optimum spraying conditions, such as, formation of desired admixed viscosity, formation of desired particle size, formation of desired sprayed fan shape, and the like.
Accordingly, in order to spray liquid coating formulations containing supercritical fluid as a diluent on a continuous, semi-continuous, and/or an intermittent or periodic on-demand basis, it is necessary to prepare such liquid coating formulations in response to such spraying by accurately mixing a proportioned amount of the coating formulation with the supercritical fluid. However, the compressibility of supercritical fluids is much greater than that of liquids. Consequently, a small change in pressure or temperature results in large changes in the density of the supercritical fluid.
The compressibility of the supercritical fluids causes the flow of these materials, through a conduit and/or pump, to fluctuate. As a result, when mixed with the coating formulation, the proportion of supercritical fluid in the resulting admixed coating formulation also correspondingly fluctuates instead of being uniform and constant. Moreover, the compressibility of liquid carbon dioxide at ambient temperature is high enough to cause flow fluctuations to occur when using reciprocating pumps to pump and proportion the carbon dioxide with the coating formulation to form the admixed coating formulation. This particularly occurs when the volume of liquid carbon dioxide in the flow path between the pump and the mixing point with the coating formulation is too large. The fluctuation can be promoted or accentuated by any pressure variation that occurs during the reciprocating pump cycle.
In an embodiment discussed in a number of the aforementioned related patent applications, (U.S. application Ser. Nos. 218,896 and 218,910) an apparatus is disclosed for pumping and proportionating a non-compressible fluid, i.e., a coating formulation with a compressible fluid, liquid carbon dioxide, for example, in order to prepare the ultimate mixture to be sprayed comprised of the coating formulation and the carbon dioxide in its supercritical state. In that embodiment, volumetric proportionating of the coating formulation stream and the liquid carbon dioxide stream is carried out by means of reciprocating pumps which displace a volume of fluid from the pump during each one of its pumping cycles. One reciprocating pump is used to pump the coating formulation which is slaved to another reciprocating pump which is used to pump the liquid carbon dioxide. The piston rods for each pump are attached to opposite ends of a shaft that pivots up and down on a center fulcrum. The volume ratio is varied by sliding one pump along the shaft, which changes the stroke length.
However, liquid carbon dioxide is relatively compressible at ambient temperature, the temperature at which it is typically stored in a pressurized container. Such compressibility may undesirably cause fluctuations and oscillations of the amount of carbon dioxide that is present in the admixed coating formulation that is to be sprayed. This occurs due to the incompatible pumping characteristics of the relatively non-compressible coating formulation and the relatively compressible liquid carbon dioxide. With the coating formulation, pressure is immediately generated in the reciprocating pump as soon as its volume is displaced. Inasmuch as the liquid carbon dioxide is substantially compressible, a larger volume is needed to be displaced in order to generate the same pressure. Because mixing occurs when the flow of the coating formulation and of the liquid carbon dioxide are at the same pressure, the flow rate of carbon dioxide lags behind the flow rate of the coating formulation.
This fluctuation is accentuated if the driving force operating the pump varies during the operating cycle, such as an air motor changing direction during its cycle. Thus, if the driving force declines, the pressure in the coating formulation flow declines even more rapidly, due to its non-compressibility, than the pressure in the liquid carbon dioxide flow, due to its being compressible.
Accordingly, the pressures generated in both flows may be out of phase during the pumping cycle, such that the proportion of carbon dioxide in the mixture to be sprayed also varies. This fluctuation is made even more severe if cavitation also occurs in the carbon dioxide pump due to vapor formation as the pump fills.
While some of these fluctuations and problems have been suppressed by refrigerating the liquid carbon dioxide to low temperatures such as below 10.degree. C., and even below 0.degree. C., prior to its entering the reciprocating pump, a need still existed to avoid substantially all inaccuracies that may be present in the proportionation of the non-compressible coating formulation and the compressible liquid carbon dioxide to form the desired admixture. Indeed, a need existed to provide a means to accurately proportion any compressible fluid with a non-compressible fluid.
That need was met in the aforementioned related patent application, U.S. patent application Ser. No. 413,517, filed Sept. 27, 1989, wherein apparatus and methods are disclosed for accurately and continuously providing a proportionated mixture comprised of non-compressible fluid and compressible fluid for spraying upon a substrate to be coated, relying particularly upon mass proportionation, to obtain the desired mixture of the compressible and non-compressible fluids.
Generally, the apparatus of U.S. patent application Ser. No. 413,517 comprises:
a) means for supplying substantially compressible fluid;
b) means for measuring the mass flow rate of the substantially compressible fluid;
c) means for generating a signal in response to the measured mass flow rate of the substantially comressible fluid;
d) means for supplying substantially non-compressible fluid;
e) means for controlling the flow rate of the substantially non-compressible fluid responsive to the signal generated in (c); and
f) means for forming a mixture of the measured compressible fluid and the controlled non-compressible fluid.
The broadest method disclosed in that application for forming a mixture of a substantially compressible fluid and a substantially non-compressible fluid in a predetermined proportion includes:
a) supplying substantially compressible fluid;
b) measuring the mass flow rate of the substantially compressible fluid;
c) generating a signal in response to the measured mass flow rate of the substantially compressible fluid;
d) supplying substantially non-compressible fluid;
e) controlling the flow rate of the substantially non-compressible fluid responsive to the signal generated in (c); and
f) forming a mixture of the measured compressible fluid and the controlled non-compressible fluid.
As used in that application and as used herein the phrase "compressible fluid" is meant to include a material whose density is affected by a change in pressure to an extent of at least about 2 percent.
Specifically, the mass flow rate of the compressible fluid is continuously and instantaneously measured. Regardless of what that flow rate is and whether or not it is fluctuating as a result of, for example, being pumped by a reciprocating pump or regardless of the state in which such compressible fluid is in, that mass flow rate information is fed to a signal processor on a continuous and instantaneous manner. Based on that received information, the signal processor in response to the amount of compressible fluid that has been measured, controls a metering device which controls the rate of flow of the non-compressible fluid. The non-compressible fluid is then metered in a precise predetermined proportion relative to the compressible fluid flow rate such that when the compressible and non-compressible fluids are subsequently mixed, they are present in the admixed coating formulation in the proper proportions.
By measuring the mass flow rate of the substantially compressible fluid, and then controlling the amount of non-compressible fluid in response thereto, the problems associated with the compressibility of the compressible fluid and the problems associated with phase changes of the compressible fluid, such as vaporization or condensation, are substantially eliminated. Any fluctuations or oscillations present in the flow of the compressible fluid are instantaneously measured and are compensated by controlling the amount of non-compressible fluid to provide the prescribed proportionation for the desired mixture. In contrast to past techniques, the present embodiment involves the metering, i.e., controlling the flow rate, of only one fluid, namely, the noncompressible fluid. The flow rate of the compressible fluid is not controlled, but rather only measured, from which measurement the prescribed amount of non-compressible fluid is correspondingly adjusted to provide the desired proportionation. This allows for total flexibility of the system and provides for a simple and effective means for producing the desired proportionated mixture of compressible and non-compressible fluids.
The apparatus and methods discosed in application Ser. No. 413,517, however, are particularly effective and specifically focused for producing the desired proportionated mixture of compressible and noncompressible fluids on a relatively large scale, continuous basis. The inventions disclosed in that Application are most suitable for substantially large industrial facilities wherein the substrate to be coated typically is transported on a conveyor system past one or more spray guns, which may be stationary or moving, to be sprayed by the apparatus disclosed therein. Such systems, and the like, may generally be used to coat automobile parts; electric motors; containers; pipe; coil steel, paper, fabric and other materials that are coated as they are rewound; plywood; porcelain enameling stove parts; adhesive on panels and honeycomb for laminating; sheet metal parts such as washers, dryers, refrigerators and the like; automotive bodies; furniture; case goods; and heavy machinery.
There are applications, however, wherein the continuous apparatus of the scale and sophistication envisaged in application Ser. No. 413,517 cannot meet on a practical and economical manner. Thus, for example, the automobile refinish industry, and small "end-use" shop and field spraying operations, and the like, where "economics-of-scale" dictate low cost equipment and a simple mode of operation, cannot effectively utilize the type of equipment disclosed in application Ser. No. 413,517. What is needed is a simple semi-continuous method and apparatus, which is portable and small in scale; for example, about, but not restricted to, a fluid output of about 0.01 to 0.2 gallon per minute and a total capacity of about 0.1 to 1 gallon in the spraying system.
Although smaller in size, this system still must be able to feed, accurately proportion, pressurize, heat and mix a plurality of fluids, particularly one or more compressible fluids with one or more non-compressible fluids, and then be able to spray such mixed, heated and pressurized fluids through a spray gun. Most preferably, this relatively small-scale, semi-continuous unit should be able to accurately proportion, pressurize, heat and mix a coating material with a supercritical fluid, such as supercritical carbon dioxide, and spray such a mixture at supercritical conditions. Moreover, the apparatus should also be able to avoid settling of the contents of the mixed fluids such as when preparing, for example, a pigmented coating system; be easily cleaned when color changes are necessary; minimize the amount of solvent emissions to the environment; have a minimum of dead space; provide for circulating the coating fluid continuously through the spray system and gun to maintain precise temperature and pressure control; and have a minimum of moving parts requiring seals from which leaks lay occur.